1. Field of the Invention
This invention relates to differential-pressure measuring apparatus adapted for use with industrial processes. More particularly, this invention relates to improvements in such apparatus for preventing damage due to overrange pressure conditions.
2. Description of the Prior Art
Differential pressures in industrial processes are measured for a wide variety of purposes. The largest application probably is that of flow-measurement, wherein the instrument measures the differential pressure produced across an orifice plate in a flow pipe in order to develop a fluid flow-rate signal. There are however many other applications, such as measurement of pressure drops across pumps, valves or the like.
For many years, differential-pressure measuring apparatus typically comprised transmitters of the force-balance type, such as shown in U.S. Pat. No. 3,564,923, issued to H. W. Nudd, et al. In recent years, transmitters which do not employ force-balance techniques have grown in importance. U.S. Pat. No. 4,165,651 (E. O. Olsen et al) shows one such design having important advantages.
Differential-pressure instruments typically include a sealed interior pressure chamber containing a fill-liquid. A pair of flexible diaphragms are mounted at opposite ends of the pressure chamber to apply an input differential pressure to the fill-liquid. In one advantageous arrangement, the pressure differential applied to the fill-liquid is sensed by an IC strain-gauge chip mounted within the sealed chamber. The present invention is disclosed hereinbelow as embodied in such a strain-gauge type of instrument, but it will be clear that the invention can be employed with other types of instruments.
Differential-pressure instruments need special protection from overrange pressure conditions, especially to prevent damage to the commonly used pressure-sensing devices such as an IC chip as described above. Overrange pressures develop in various ways, frequently by operator error. For example, when an instrument is being placed into or taken out of service, the operator may inadvertently allow full static pressure (e.g. 2000 psi) to be connected to only one side of the instrument. With an instrument designed to handle differential pressures of, say, 20 psi maximum (as in flow measurements), the application of 2000 psi to only one side will, unless effective protective measures are taken, almost certainly destroy the instrument.
One approach to providing protection against over-range pressures is to employ back-up plates for the flexible diaphragms, as disclosed for example in the above-mentioned Olsen et al patent. In such a design, each flexible diaphragm is arranged to bottom on the corresponding back-up plate in response to an overrange differential pressure. This prevents further movement of the diaphragm, and thus prevents transfer of any additional fill-liquid from the region adjacent the bottomed diaphragm.
Although this approach can be effective in certain applications, such as in the above Olsen et al patent, where the instrument sensor effectively responds to the force developed by a range diaphragm, problems are encountered when this approach is applied to instruments wherein the sensor employs a third diaphragm or plate which deflects with differential pressure thus allowing the slack diaphragm to deflect until it bottoms on its back-up plate. For example, because the volume of the fill-liquid expands and contracts with changes in instrument temperature, the amount of differential pressure required to effect bottoming of the diaphragm on its back-up plate varies correspondingly. Prior designs of apparatus for achieving overrange protection also have been relatively costly to manufacture, and particularly have not been well adapted for use in a multi-model family of essentially identical instruments for covering the full range of spans needed for industrial processes.